Optical telecommunication components that work at the optical channel level such as Dynamic Channel Equalizers, Wavelength Blockers, Wavelength Selective Switches and similar devices often have alignment tolerances for optical beams of 50 μRad or less. Moreover, this alignment must be maintained over wide temperature swings, for example a temperature range of 75° C. In addition, this alignment must be maintained while sustaining large accelerations in any orientation. However, thermal conditions in the operating environment can cause optical beams to become misaligned in typical channel based telecommunications components. While the alignment of optical beams is usually stable at a fixed temperature, these devices will be exposed to wide temperature swings which can introduce a variety of types of errors in optical alignment, although the magnitude of error associated with each mechanism typically differs. These mechanisms will, in general, cause the pointing of the optical beams to vary as the optical components are translated and twisted with respect to their nominally aligned positions. One mechanism which introduces error is thermo-elastic distortion of the optical housing that holds the optical components (lens, mirror, grating, waveplates, MEMS mirror arrays, etc.) in place, and can cause misalignment in the pointing of the beam impinging on the grating and the twisting of the grating itself. Both distortions change the incident angle of the light on the grating that changes both the grating dispersion and the position of the undiffracted and diffracted beams. This type of misalignment results in a change in the positions of the optical channels on the MEMS element.
Additional much smaller error mechanisms result from the change with temperature of the refractive index of the air and the glass making up the prisms used for the beam expander. The changes in the air refractive index will steer the diffracted beams because the wavelength of the light varies with the air refractive index, which in turn causes the grating dispersion to vary. As the prisms refractive index varies with temperature beams exiting the beam expander will be steered and so the incident angle at the grating will vary. Moreover, a small error can result from variation in the grating pitch caused either by thermal expansion of the grating substrate or by thermo-elastic distortions of the metal grating mount applying a stress on the grating which distorts the grating.
Although the error mechanisms discussed above in the positioning of the optical channels at the MEMS mirror array present unique challenges, the need to compensate for thermal variation has been recognized in diverse areas. Precision machine tools and clocks have for hundreds of years been designed with compensation mechanisms to correct for the changes in physical length that can lead to misalignment in machine tools and period change of mechanical oscillators as the temperature changes. This has, historically, been accomplished by any of several approaches, each of which has numerous limitations, particularly in the field of telecommunications.
The oldest technique is to use materials with different coefficients of thermal expansion attached in such a way that the lengthening of one part just compensates the lengthening of a complementary part. However, selecting materials with a complementary coefficient of thermal expansion limits the choices to materials that may be used and these materials may be difficult to use in low cost manufacturing. In addition, the materials used to make the optical components (gratings, lenses, prisms, window and optical crystals) may then have large coefficient of thermal expansion mismatches between themselves and the metal housing.
A second technique has been to temperature control the mechanism by placing it in an oven and then using active feedback control to keep the temperature constant. Maintaining a constant temperature, however, requires either setting the oven temperature above the highest temperature ever expected or including a cooling element along with the oven. Both of these approaches will require the consumption of large amounts of power. In addition, continually operating the device at a high temperature will accelerate the natural aging mechanisms of the components and epoxy joints between components.
A third approach is to select materials with very low thermal expansion coefficients. Such a passive compensation scheme will only work if the nature and magnitude of the disturbance to the optical plant is known and constant. In addition, such schemes typically suffer from most of the ills of the complementary approach described first above.
As a result none of the historical approaches has proven well suited to the demands of the telecommunications industry, where precision, low power consumption, long life, high reliability and low cost are considered desirable in some or all embodiments.